In order to drive a hydraulic traveling motor in a hydraulic drive-type travel vehicle, oil, discharged from a hydraulic pump, is switched by a direction changeover valve and is subsequently supplied to one of the two ports of the hydraulic motor. Return oil flows from the other port of the hydraulic motor to a tank via the direction changeover valve, and rotationally drives the hydraulic motor in one direction, for example the forward direction or the backward direction, or in the other direction to drive a traveling body.
In the meantime, when the hydraulic drive type travel vehicle goes downhill, the drive wheels are rotated by gravity acting on the vehicle, and the hydraulic motor is inversely driven by the drive wheels, so that the hydraulic drive type travel vehicle descends rapidly. In such a case, the hydraulic motor needs to be braked and stopped.
Accordingly, two of the present inventors and another proposed a drive device for a hydraulic motor in an unpublished Japanese Patent Application No. 7-065718. The unpublished proposal will be described with reference to FIGS. 8 and 9. Specifically, a directional control valve (a direction changeover valve) is a pilot pressure changeover type, and a pilot pressure supply valve is provided for supplying a pressure of a main circuit as a pilot pressure to a pressure receiving element of the directional control valve. When the hydraulic motor is inversely driven by an external force, such as a traveling body and the like, the pressure of the main circuit is lowered, whereby the pilot pressure is lowered so that the directional control valve is switched to its neutral position to give a braking force to the hydraulic motor. When the directional control valve is in its neutral position, a predetermined pressure is supplied to the main circuit, thus securing the pilot pressure and preventing cavitation at the time of braking and stopping.
In other words, as shown in FIG. 8, a pilot pressure supply valve 37, which is switched to its first position G or its second position H in accordance with a command from a final control element 38, is provided. A directional control valve 30 is switched to its first position B or its second position C upon receipt of the pressure of the first main circuit 21 or the pressure of the second main circuit 22 of a hydraulic motor 24, either of which passed through the pilot pressure supply valve 37. When the directional control valve 30 is switched to its first position B, pressurized oil discharged from the hydraulic pump 20, flows into the first port 25 of the hydraulic motor 24 via the first main circuit 21, and return oil flows from the second port 26 into the tank 27 via the second main circuit 22, the directional control valve 30, and the backpressure valve 35, so that the hydraulic motor 24 is rotationally driven in one direction (indicated by the arrow D). Also when the directional control valve 30 is switched to its second position C, the hydraulic motor 24 is rotationally driven in the other direction (indicated by the arrow E) in the same way as above.
As described above, when the hydraulic motor 24 is inversely driven by an external force, such as a traveling body and the like, while being rotationally driven, the pressure of the first main circuit 21 or the second circuit 22 is reduced so that the directional control valve 30 is switched to its neutral position A, where the first check valve 31 and the second check valve 32 prevent pressurized oil from the first port 25 and the second port 26 of the hydraulic motor 24 from flowing into the tank 27, so that the hydraulic motor 24 is braked and stopped.
Meanwhile, when the directional control valve 30 is in its neutral position A, the pressurized oil, discharged from the hydraulic pump 20, flows into the tank 27 via the backpressure valve 35 and thus the discharge pressure is equal to the set pressure of the backpressure valve 35. The pressurized oil is supplied from the first check valve 31 and the second check valve 32 to the first main circuit 21 and the second main circuit 22 so that the pressures of the first and second main circuits 21 and 22 do not change over to negative pressures at the time of braking and stopping, whereby cavitation never occurs.
However, in the aforesaid unpublished proposal, the return oil from the hydraulic motor 24 always flows into the tank 27 via the backpressure valve 35. Hence, the set pressure of the backpressure valve 35 is applied to the return oil even at the time of drive by the hydraulic motor 24, thereby raising the pressure of the pressurized oil, discharged from the hydraulic pump 20, by the same pressure as the set pressure. Accordingly, horsepower to drive the hydraulic pump 20 in a manner corresponding to the above raised pressure is needed, which leads to a lowering of the drive efficiency. When the hydraulic motor 24 is used at a low speed rotation, there is little loss in horsepower since cavitation never occurs even when the set pressure of the backpressure valve 35 is low. However, when the hydraulic motor 24 is used at a high speed rotation, there is the following disadvantage. When a set pressure is low even if the backpressure valve 35 is used, as shown in FIG. 9, the backpressure is in short supply at the time of braking and stopping, thus causing cavitation. Therefore, the set pressure of the backpressure valve 35 should be increased. With the increase of the set pressure, more drive horsepower is needed, thereby increasing the loss in horsepower and lowering the drive efficiency. Accordingly, the usage of rotating the hydraulic motor 24 at high speed, thereby gaining power, is difficult.